Production of alumina



Aug. 2, 1960 F. LAlsT PRODUCTION oF ALUMINA Filed Oct. 22, 1956 mam IIT

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4222344 uc3m@ IIII I I INVENTOR ATTORNEY S FREDERICK LAIST BY m44,

I SOJHTO atcS PRODUCTION F ALUMINA Frederick Laist, Los Angeles, Calif., assignor to The Anaconda Com an New York N.Y. of Montana p y, a corporation Filed (let. 22, 1956, Sel'. No. 617,614

Claims. (Cl. 2li- 143) This invention relates to the A,production of alumina from clays. More particularly the invention is concerned `with the production from clay of a high grade, substan tially silicaand iron-free `alumina suitable as feed in the electrolytic aluminum reduction process.

Bauxite, the principal ore of alumina (A1203), has heretofore been, and is presently, the main source of alumina as feed in the electrolytic production of aluminum. Clay has long been recognized as another possible source of high grade alumina, and various proposals have heretofore been advancedfor producing such alumina from clays. ln clay, alumina is combined with silica (SiO2), generally in the form of a hydrous silicate of `alumina (eg. Al2O3-2SiO22H2O). `Clays further contain `varying amounts of other constituents, 4frequently re- `felred to as impurities, such as excess silica, iron (Fe) compounds etc. The present invention contemplates an improved method of` producing a-high grade alumina product substantially free of silica and iron, from clays containing varying amounts of silicaand iron.

The method of the invention involves two stages of htreatment in the rst of which silica is removed from `the clay and a crude alumina containing ferrie oxide (FezOg) is produced, and inthe second Vof which the ferric oxide Lis A,eliminated and a high'grade alumina product is produced. Basically, `the rst stage treatment involves .calcining the `clayat an Aelevated temperature, say at least l0`00 V(and preferably MOO-.13009 F), to render the alumina soluble indilute hydrochloric :acid (HC1). The calcinedclay is leached with an aqueous solution of hytirochloric` acid of .a concentration of 4about 20% `HC1 to dissolve aluminum and iron, as chlorides, 4leaving practically all of thesilica inthe'insoluble leach residue. 1-Folloyving .ltratioin or .othersuitable solids-liquid separatQrystep, the solutionof .aluminumand iron chlorides .,isfccncentrated by evaporation to produce "a .crystal slurry or magma of. aluminum p and viron chlorides l (ifslQlB:oHZQfl-EeClgdHgO).-. Thisy crystal slurry, `after l suitable conditioning iirnecessary or desired, is-calcined at' an elevated temperature, say atleast 15002"` `F. (and pref- P 1crably.17.00-49009.l?. ),;inthe course of which the chlorides .are r( :lecomposed intofoxides with-the `evolution of hydrogen .chloride .(.hydrochloric acid gas) .which is reovered forrreuse -Tinthe treatment `of further -calcined i `Il;,e-calcine `is a practically silica-freecrude alu-mina r usuallycontaining about 85% A(,plusorrninus 5%) A1203 .(plus or `minus 5%) Fe-03, .depending on the qualityof the clay feed, and constitutes :the feed of the second stage treatment.

-fjlheprude alumina, .consistingprincipallyof the oxides ofaluminummnd iron l(eg. in` the relative proportions of .around 6 to l), is sintered in the presence of sodium vcarbonate .or soda ash (.Na2CO3) i at a temperature of atleast-1500? F. (andpreferably1700-1900" F.), in `the coursepf which-.the `alturlinumoxide is `converted into water-solublesodium aluminate (NaAIOZ) While the iron is retained in theresultng calcine as insoluble ferrie .QXitle HFor .convenience this Operation iS herein desisatent ICC nated the soda-sinter step. The sinter of this step is leached to dissolve the `sodium aluminateand the resulting solution is separated from the insoluble leach residue consisting mainly offcrric oxide. The solution of sodium aluminate is treated with carbon dioxidegas in the course of which aluminum hydroxide (AMOI-D3) is `precipitated and sodium carbonate is regeneratedfor reuse in the sodasinter step. The precipitated aluminum hydroxide, after washing if necessary or desired, is dehydrated by calcination at an elevated temperature, say at least 1600 Ft (and preferably 1700-2100" E); `the resulting calcine being a high grade, substantially silicaand iron-free alumina suitable `as feed to the pot line of :an Aelectrolytic aluminum reduction plant.

The purpose of the- (first stage) hydrochloric Aacid extraction treatment is toproduce a crude alumina product, free of silica, for a succeeding purication treatment. Metallurgically the alumina and iron in the `calcined clay are soluble in 20% hydrochloric acid, while silica is insoluble. The chlorides of aluminum and iron (after separation from the silica residue andsuitable dehydration) are decomposed by thermal treatment to yield a crude alumina product (contaminated with iron oxide in proportion to its occurrence in the clay) and hydrogen chloride gas. This gas is absorbed `in `an `intermediate wash liquor and recoveredas 20% `hydrochloric acidfor reuse in the treatment of further calcined clay. The (second stage) soda-sinter purification treatment requires the absence of `silica which would be solubilized in the soda-sinter step. Since the silica in the initial clay is eliminated by the hydrochloric acid `extraction treatment, the crude alumina product is amenable `tothe soda-sinter treatment. Aln the soda-sinterstep, the alumina is `solubilized as sodium aluminate,'while iron remainsras insoluble ferrie oxide and is eliminatedby leaching the soda-sinter. Through the successive steps of the purification treatment, a high great alumina, free of silica Vand iron, is obtained from the sodium aluminate.

The invention will be better understood from the following description taken in conjunction `with the accom panying drawing, in which the single gure is adiagrammatic flow sheet `of the main 'features the rst and second stage treatments of the method of the invention.

The raw or green clay isprepared for calcination in appropriate `steps Aof Asizing `and conditioning. Thus, the clay maybe iirst reduced ,to Vlumps of about 3A inch iu size, in a plurality of stages of size-reduction. The sized material is 4advantageously conditioned in .a pug mill, and then passed through a cutter or shredder. "The thus-conditioned, clay is dried and calcined in' any suitable `type of kiln, preferably' at a temperature o'f about 1200` F. with a retention period of about `ll/z hours at ,that temperature. If the clay is sticky, a certain proportion of dried or calcined clay may be mixed `therewith and circulated through the drying and calcining .kiln Dust from thecalcining kiln may be collected and `returned to the conditioned step `following sizing, `and a product cooler may be operatively associated With the kiln. The calcined clayis ground to a nominal particle size of about mesh Tyler screen series, advantageously in a 2-stage open circuit ball mill.

The ground calcined clay is leached with `.hydrochloric V` cyclone vor filter orY a combination thereof, to produce a clarified pregnant solution of aluminum chloride and iron chloride, filter wash liquor and the washed solid residue; the wash liquor being delivered to the HC1 absorption tower and the solid residue discharged to waste.

The pregnant solution of aluminum chloride and iron chloride is evaporated in any suitable manner, as for example by vacuum evaporation ,or the like, to produce a mixed crystal slurry or magma and an aqueous condensate.

orator will contain sucient hydrochloric acid to warrant its recovery and return to the process, e.g. as filter wash liquor or (if sufficiently concentrated) as aqueous absorbing medium in the HC1 absorption tower.

When, as usual, the crystal slurry contains free. hydrochloric acid, the aqueous condensate from the evap- The mixed crystal slurry (usually around 25-30%n'" solids) from the evaporator is treated in a classifier (e.g.

' bowl classifier) with the overflow thereof being returned to the evaporator. The drained crude crystals (usually around 50-60% solids) may advantageously be conditioned in apug mill preparatory to delivery to the chloride decomposition kiln, as for example in a pug mill type kiln feeder. 'I'he kiln is provided with a product cooler, dust collector and means for returning to the kiln dust and v(if desired) a certain proportion of the calcined .t

ride crystals are dried and calcined, preferably at a temperature of about 1700J F., for about 1 hour, in the t course of which the chlorides are decomposed into aluminum and iron oxides and hydrogen chloride gas. The

Vcalcined product of this heat-treatment constitutes ther crude alumina whichis purified in the second treatment stage of the present method. The gaseous product of the chloride decomposition kiln is preferably passed through suitable dust recovery equipment; the dust being returned to the kiln and the dust-free gas delivered to the HC1 absorption tower. Inthe absorption tower, hydrogen chloride gas is condensed in an aqueous absorbing medium The crude alumina produced in the rst stage is ground,

say to nominal `100 mesh, and the ground product issubjected to a sintering heat-treatment in the presence of sodium carbonate. The sodium carbonate may advantageously be mixed with the crude alumina in the form of a concentrated aqueous solution recovered from the' process as hereinafter described. Make-up sodium carbonate maybe added dry to and mixedwithA the sodasinter charge. The sintering operation is preferably car- A ried out at a temperature within the range of 1700 and 1900 F. with a retention period of about one hour at that temperature in any appropriate type of kiln, preferably provided with a pug mill type feeder, cyclone dust collector and return, and product cooler.

InV the course of sintering substantially all of the alumina is converted to water-soluble sodium aluminate, while the iron remains in the sinter as insoluble ferric oxide. The sintered product is leached, preferably with hot wash product. In this kiln, the mixed aluminum and iron chlois delivered to a carbonation tank for treatment with carbon dioxide gas which may advantageously be obtained from the aluminum hydrate calcining kiln and/ or from a natural gas burner or the like.

In the carbonation tank, sodium aluminate reacts with carbon dioxide to form a precipitate of aluminum hydroxide and water-soluble sodium carbonate. The slurry from the tank is delivered to a hydrate thickener (or other suitable solids-liquid separator) from which the aluminum hydroxide is discharged in the underflow and the sodium carbonate in the overliow. By evaporation the sodium carbonate overflow is concentrated to a suitable degree for return to the'soda-sinted treatment of crude alumina. The aluminum hydroxide underflow is washed and tiltered, and the wash water is utilized in the leaching of soda-sinter product.

The washed aluminum hydroxide is calcined in-any suitable type of kiln preferably provided with a cyclone dust collector, calcine return means and product cooler, at a temperature within the range of 1700 Vand 2100 F. and preferably about l800 F. with a retention period of about one hour at that temperature. The kiln is preferably gas-fired, or may even be oil-fired, rather than coalred to avoid ash contamination. The calcine consists of purified alumina, practically silica-free and analyzing (dry weight) 99+%1Al203 and insignicant amounts of Fe2'O3 and NazO. Dustfrom the gaseous product (kiln gas) of the calcining kiln is recovered in a cyclone `and may be returned to the kiln, and the carbon dioxide in the residual gas (after passing the gas through a scrubber tower) is utilized in the aforementioned carbonation tank.

Vsuch as an aqueous solution of hydrochloric acid. kThis water from subsequent clarication operations to dissolve the sodium aluminate, which is separated from the insoluble sinter residue (largely VFe203) by ltration. lf

, desired,ethe filtrate may be further clarified in a polishing filter or the like. "Ihe clarified sodium aluminate solution The sodium alum-inate produced iu the soda-sinter kiln has a Vhigh degree of water-solubility. The reaction during leaching of the soda-sinter productY is strongly exotherrnic and leaching is preferably carried out at a temperature as near the boiling point as practical and at least above F. t

Optimum operating results are obtained by observing the aforementioned preferred temperature ranges of the various calcining operations. Thus, clay calcined at temperatures below l000 F. does not give good extraction of alumina in 20% hydrochloric acid. On the other hand, at calcining temperatures of 1500 F. and higher, alumina and silica fuse and form a xation compound insoluble in 20% hydrochloric acid. Accordingly, in practicing the invention, it is desirable to avoid these unfavorable temperatures, and hence the clay calcinug kiln is preferably operated within the temperature range of 1100 and 1300 F.

The-chloride `decomposition kiln should be operated at a temperature above 1500 F. to the amount of residual chlorine in the crude alumina. At a calcining temperature of 17009 F. the crude alumina is substantially free of chlorine (e.g. 0.1% or less). Hence, the chloride decomposition kiln is operated with-I in the temperature range of 1700 and 1900 `F. for optimum results. t

The soda-sinter step should be carried Iout at a tern-Y perature of at least 1600 F. since lower-temperatures 'd o not. give good conversion to sodium aluminate.

Sintering at a temperature much above 1900 F. gives a more glazed, less granular product of relatively lower water-solubility. Hence, the preferred temperatureof the soda-sinter step is within the range of 1700 and 1900 F., within which temperature range the color of the product is a very pale green olf-White.

. lf the temperature of calcination of the aluminum hydrate is too low, that is lower than 1600 F., the calcined alumina is hygroscopic. Optimum results are obtained within the temperature range of 1700V andv 2100 F. Calcining temperatures near the higher-end of the range, and even above 2100 F., make possible desirable particle size control.

Depending on current and local economics, the` sodium compound required for practicing` the invention may be supplied by sodium hydroxide rather than sodium carbonate as hereinbefore described. If the sodium compound is introduced into the circuit as sodium hy.: droxide, it will be necessary to carbonate it before inclusion in the soda-sinter charge. This may be done by silica-free carbon dioxide gas recovered from the hydrate calcining kiln, from a gas-tired steam `boiler stack of the plant, or the like. Both the sodium hydroxide and the hydrochloric acid, required in practicing the vention, may be locallymade by electrolysis of sodiuin chloride as conventionally practiced, and carbon dioxide for carbonating the sodium hydroxide may be supplied as aforesaid.

Various types of equipment may be used in carrying out the individual steps of the method, and apparatus herein specifically described orindicated on the ofw sheet are illustrative and in no manner restrictive. The equipment in the aluminum chloride-ferrie chloride and free hydrochloric iacid circuits must be resistantto corrosive attack, and to this end `presently available rubber, plastic and ceramic types of materials-of-construction are adequate. Aside from pumps, the` movements of liquid are relatively slow, and where necessary rubberlined equipment is readily available therefor. The use of` expensive special alloysis a minor item of equipment cost. Drying, calcining and sintering may `advantageously be carried out in rotary kilns, although other types of kilns and furnaces may be used; Leaching is carried out at tempenatures that `do not require autoclaving, and' hence open tanks provided With mechanicall agitation are entirelysatisfactory. Filtration isprefe'rably (but not necessarily) carried out in vacuum type equipment, preceded where practical by thickening or clarification to reduce as far as possiblethe areas of ilter surface required. Thickening may be carried out.

in "wet cyclones, thickeners, clariers or the like. Evaporation is preferably carried out in vacuum type evaponators. i

The following example illustrates a practice of the invention with an Idaho clay containing (as mined) Y25- 30%.free moisture (H30), and analyzing (drylwe'ight) aboutr26% A1303, aboutr5.7% Fe303, about 54% Si03;

:the balance being mainly combined wate'r. The'example :is purely illustrative and not. restrictive of the invention.

The dried clay was calcined for 11/2 hours at a temperature of 1150 F., with a calcining weight loss of about 12%. The calcined `clay analyzed about 30% A1303, about 6.5% Fe2O3 and the balance mostly SiOZ and was ground to nominal 65 mesh (i.e. 3-5% on and the balance through 65 mesh) with 11% by weight plus 100 mesh and 62% by weight minus 200 mesh.

The calcined clay was leached with mechanical agitation for l hour at a temperature of about 210 F. with an aqueous solution of hydrochloric acid of 20% HC1 concentration; about 65 parts by Weight of 100% HC1 being initially present for each 100 parts by Weight of calcined clay. After solids-liquid separatory steps of thickening, clarification and tiltering, 82% of the A1303 in the calcined clay was recovered in the pregnant solution as AlCl3. By analysis, the pregnant solution contained 75 g./l. (grams per liter) of A1203, 7.8 g./l. Fe and 5.4 g./l. of free HC1. The total loss of hydrochSk'ric acid in the acid extraction treatment was about 2. o.

The pregnant solution of aluminum and iron chlorides was subjected to vacuum evaporation, followed by washing and draining of the resulting crystal slurry. The crystal slurry as fed to the chloride decomposition kiln analyzed 56% AlCl3-6H2O; 7.5% FeCl36H3O, 10% A1Cl3, 1.5% FeCl3 and 25% free H30, being thus about 50% solids. The kiln was maintained at a temperature of 1700-1800 F. and the detention period of the charge at that temperature was 1 hour. The calcine (crude alumina) analyzed about 84% A1303, about 16% lie-303, less than 0.1% residual chlorine, and negligible silica.

About 97% of the hydrogen chloride in the `gaseous product` of the chloride decomposition kiln was recovered in the 20% HCl effluent of the HC1 absorption tower. The over-all loss of HC1 was about 4%, and this amount of make-up acid `(as 100% HC1) was included in the 20% `HC1 leach liquor for the chloridizing leachfing ofthe calcined clay. s

i 'Ihe crude alumina was ground to nominal 100 mesh, and lmixed with recycled sodium carbonate solution and make-up sodium carbonate, in the proportion by :weight of about 40% crude alumina, 50% Na3CO3 in recycled solution and 10% make-up Na2CO3. This mix- `ture was sintered at a temperature of 1900 F. for

1 hour, in the course of which the alumina was converted to sodium aluminate while iron remained unchanged in the sinter as Fe303. `The sinter product analyzed 42.3% A1203, Nago, `Nazcog (free) and Fe, and was ground to nominal 100 mesh. The ground sinter was leached with water (iilter washrwater) at a temperature of about 210 F. `in about one-half hours time. M j

`The sodium aluminate` filtrate was carbonated by carbon dioxidein the scrubbed gaseous product of a naturalgasburner. The precipitatedaluminum hydroxide `was. thickened, washed andcalcined at a temperature of 11800F. `to produce the final high grade alumina, 75%

of` thejalumina' content` of the originalw clay being `re- The thickener ,overflow and The sodium carbonate liquors were concen- `the feed` to electrolytic reduction` furnaces. t

i I claim:

.1. VIn the method of producing substantially silica-free -and iron-free alumina from anriron-containing clay in zwhiclLaniron-containing crude alumina substantially 4free of silica is ,first producedby calcining the clay, leaching the calcine with hydrochloric acid, separating the siliceous residue from, the, leach solution, and crystallizing aluminum; and iron chlorides from the leach solution,

the improvement which comprises subjecting saidchloride crystals to calcination at a temperature within the range of 1500 and 1900 F. and thereby producing an iron-containing crude alumina substantially free of both silica and residual chloride, sintering said crude alumina in the presence of sodium carbonate at a temperature in excess of l5 00 F. and thereby converting the alumina to sodium aluminate while retaining the iron as oxide in the resulting sinter product, separating the sodium aluminate from the iron' oxide residue associated therewith by leaching with an aqueous medium, recovering substantially silica-free and iron-free alumina from the aqueous solution of sodium aluminate, recovering substantially chloride-free sodium carbonate from the residual aqueous leach solution, and recycling the recovered sodium carbonate for sintering with a further quantity of iron-containing crude alumina.

2. The improvement according to claim 1 in which the crude alumina is sintered in the presence of sodium carbonate at a temperature within the range of 1700 Aing sinter product, separating the sodium aluminatefrom the iron oxide residue associated therewith by leaching with an aqueous medium at a temperature exceeding 170 F., recovering substantially silica-free andViron-free alumina from the aqueous solution of sodium aluminate, recovering substantially chloride-freesodium carbonate from the residual aqueous leach solution, -and recycling the recovered sodium carbonate for sintering with a further quantity of iron-containing crude alumina.

4. In the method of producing substantially silica-free and iron-free alumina from an iron-containing clay 'in which an iron-containing crude alumina substantially free of silica is first produced by calcining the clay, Yleaching the calcine with lhydrochloric acid, separating the siliceous residue from the leach solution, and crystallizing aluminum and iron chlorides from the leachA solution, the improvement which comprises subjecting said chloride crystals t o calcination at a temperature Within Vrthe range of 1500 to 1900F. and thereby producing 'anA iron-containing crude alumina lsubstantially free of both silica and residual chloride, sintering 'said' crude valumina inthe presence of sodium carbonateata temperature in excess of 1500 F. and therebyconverting the alumina 'to sodium aluminate While retaining the ironas oxide in the resulting sinter product, leaching the Ysinter product with an alkaline aqueous medium 4to dissolve the sodium aluminate and yseparating the resulting leach solution from the insoluble iron oxide residue, treating :the separated leach solution with carbon dioxide to precipitate alumina therefrom and to regenerate sodium carbonate in solution, separating the alumina from the solution and washing it `free from adhering leach solution to produce a silica-free, iron-free and chloride-free Valumina for leaching a further quantity of sodium carbonate sinter product.

5; In the'method of producing substantially silica-free and iron-'free alumina from an iron-,containing clay in Which an iron-containing crude Valumina substantially freeiof silica isrst produced by calcining the clay, leaching "the calcine with hydrochloricV acid,l separating the vsiliceous :residue from the leach solution, and crystallizing aluminum andV iron chlorides from the leach solu. tion, theiimprovement Whichcomprises subjecting said chloride :crystals to calcination at a temperature. Within the range ofl 1,700 to 1900 F. and thereby producing an iron-containing crude alumina containing less than 0.1% of residual chloride'and negligible silica, sintering said crude alumina in the presence of sodium carbonate ata-temperature within the range of 1700 and 1900 F. zandthereby `converting the alumina to sodium aluminate while retaining the'ironas oxide in the-resulting sinter product, leaching the sinter product with an alkaline aquev011s `medium :at a temperature exceeding F. to

-dissolve the sodium aluminate and separating the result- .ing leach solution Tfrom the insoluble iron oxide residue, treating .the-separatedleach solution with carbon dioxide Eto precipitate'aluminatherefrom and ,to regenerate so- -diurn:carbonate in solution, separating the alumina from ithe fsolution `and washing-it free from Aadhering leach Vsolution to produce a silica-free, iron-free and chloride- -freealumina product, recovering substantially chloridexfreev sodium carbonate from the leach solution and recycling lit for sintering with a further quantity of ironcontaining crude alumina, and employing the wash vliquor lfrom the alumina for leaching a further quantity -of sodium carbonate sinter product.

References Cited in the file o f this patent i ,UNITED -STATES PATENTS 1,341,901 Hayward June 1, 1920 1,918,735 Bradley et al. July 18, 1933 1,926,744 James Sept. 12, 1933 2,376,696 Hixson et al. May 22, 1945 2,398,425 Half Apr. 16, 1946 2,408,241 v vSturbelle Sept. 24, 1946 2,413,709 Hoffman Jan. 7, 1947 ,2,440,378 `Newsome Apr. 27, 1948 2,487,076 vSharp et al. Nov. 8, 1949 

1. IN THE METHOD OF PRODUCING SUBSTANTIALLY SILICA-FREE AND IRON-FREE ALUMINA FROM AN IRON-CONTAINING CLAY IN WHICH AN IRON-CONTAINING CRUDE ALUMINA SUBSTANTIALLY FREE OF SILICA IS FIRST PRODUCED BY CALCINING THE CLAY, LEACHING THE CALCINE WITH HYDROCHLORIC ACID, SEPARATING THE SILICEOUS RESIDUE FROM THE LEACH SOLUTION, AND CRYSTALLIZING ALUMINUM AND IRON CHLORIDES FROM THE LEACH SOLUTION, THE IMPROVEMENT WHICH COMPRISES SUBJECTING SAID CHLORIDE CRYSTALS TO CALCINATION AT A TEMPERATURE WITHIN THE RANGE OF 1500 AND 1900*C. AND THEREBY PRODUCING AN IRON-CONTAINING CRUDE ALUMINA SUBSTANTIALLY FREE OF BOTH SILICA AND RESIDUAL CHLORIDE, SINTERING SAID CRUDE ALUMINA IN THE PRESENCE OF SODIUM CARBONATE AT A TEMPERATURE IN EXCESS OF 1500*F. AND THEREBY CONVERTING THE ALUMINA 